This invention relates to a pneumatic tire equipped on its tread surface with a tread pattern for reducing sounds, particularly noise, generated with rolling of the tire.
Conventionally, the noise generated with rolling of the tire includes so-called "rustling feel" which is offensive and has an aurally and psycologically unpleasant tone (hereinafter referred to as "rustling feel"), besides the pitch noise resulting from the tread pattern.
With upgrading of vehicles such as passenger cars, the requirement for reduction of noise resulting from rolling of the tire has become severer and severer recently. In other words, the reduction of not only the pitch noise described above but also rustling feel has strongly been required as the noise reduction measure of the tire.
A pitch variation method is known as the method of reducing the pitch noise among the noises described above as explained in detail in U.S. Pat. Specifications Nos. 4,327,792 and 4,474,223. In a tread pattern consisting of the repeating design cycles of a plurality of pitches having different lengths, this method disperses the peaks of the harmonics numbers having the total pitch number in one circumferential length of the tread as the primary order by frequency modulation and converts the noise to white noise. Here, the term "harmonics number" means the number of times of occurrence of vibration per rotation of the tire. If the total pitch number per one circumferential length of the tread of the tire is 64, for example, vibration occurs 64 times. Therefore, the harmonics number at this time is 64. However, when the peaks of the harmonics number are dispersed by the pitch variation method, roughness becomes remarkably greater with a higher degree of dispersibility.
This roughness is typical characteristics which increase the rustling feel described above. Therefore, the pitch variation method involves the problem in that the more it reduces the pitch noise, the greater becomes the rustling feel.
Another problem lies in that if the pitch length and number of the pitches disposed on the tread are changed at random, adverse influences are exerted on tire performance such as maneuvering stability and comfort to drive of the tire.